WASHINGTON—Treasury Secretary
Steven Mnuchin
and IRS Commissioner
Charles Rettig
defied congressional subpoenas by missing a Friday deadline to hand over six years of President Trump’s tax returns and audit records.

Their decision was expected after weeks of rebuffing requests from Rep. Richard Neal (D., Mass.), the chairman of the House Ways and Means Committee, who invoked a statute that lets him obtain any taxpayer’s returns from the Treasury Department.

“We will likely proceed to court as quickly as next week,” Mr. Neal told reporters earlier Friday, anticipating Mr. Mnuchin’s rejection of the subpoena. Mr. Neal said he didn’t see holding Mr. Mnuchin in contempt as an option. In a statement late Friday, Mr. Neal said he was consulting with lawyers about how to enforce the subpoenas.

Democrats have several procedural paths as they seek the documents. They can ask a federal court to enforce the subpoena and their prior request. They can lump this request together with other Trump administration actions to ignore or reject congressional demands for information. They could even use what are known as Congress’s inherent contempt powers to levy fines against Messrs. Mnuchin and Rettig.

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“Mnuchin has no leg to stand on in court, and he knows it,”
Rep. Don Beyer
(D., Va.), a Ways and Means member, wrote on Twitter. “This isn’t about precedent or purpose, it is about hiding Trump’s tax returns and engaging in illegal stonewalling of Congress.”

Asked earlier in the week whether he was concerned about fines, Mr. Mnuchin said he was following the advice of the Justice Department, which is expected to publish a legal opinion justifying Treasury’s withholding of the tax returns.

Mr. Trump said during the 2016 presidential campaign that he would release his tax returns but never did so, breaking a decadeslong tradition of voluntary disclosure of returns by presidential nominees of major parties. He has cited a variety of reasons, most frequently his reluctance to disclose his returns during an audit. His lawyers said in 2016 that he had been under audit continuously since his 2002 tax return and that tax years from 2009 forward were still under audit.

No law prevents him from releasing his tax returns during an audit. Since 1977, all presidents have been under mandatory audit under an Internal Revenue Service policy, and such audits didn’t prevent Mr. Trump’s predecessors from releasing their tax returns.

In his letter Friday, Mr. Mnuchin wrote that the Treasury Department is “not authorized” to provide the subpoenaed documents because it has determined that Mr. Neal lacks a legitimate legislative purpose for them. Mr. Neal has said he is trying to determine whether the IRS is adequately auditing the president who oversees the agency. Mr. Trump’s returns and audit records would shed light on that issue as well as the president’s tax strategies and any potential conflicts of interest.

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Mr. Mnuchin wrote that Treasury would provide lawmakers, if they wanted it, with more information on how presidents are audited.

It could take a while to get a case into court, and once there, the dispute could take months, if not longer, to resolve. Congress may go through several more steps, with the committee and then the full House of Representatives potentially voting to hold the administration officials in contempt and authorize lawsuits before a case is filed, said Andy Wright, who was a lawyer for House Democrats and the Obama White House.

“Part of the idea here is to build a record here to show that you have tried to bend over backwards to address legitimate concerns of the other branch while pursuing your own interests,” said Mr. Wright, now a partner at K&L Gates LLP.

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